The History of Planet Earth

PRE-TEST
1. How is the estimated age of the Earth determined? It is
determined through ____.
A.fossils
B.absolute dating
C.relative dating
D.the use of the clock

2. How do geologists separate time periods in the geologic time

scale? They separate it by _________.
A.studying fossil records
B.looking at the relative time of a fossil
C.counting the years a fossil has been preserved
D.looking at the special events that have happened in that
period
3. In which type of rocks are fossils found?
A. igneous rock
B. sedimentary rock
C. metamorphic rock
D. all of the above

4. Which of the following is/are characteristics of a marker fossil?

I. abundant III. widespread
II. Ishort lived IV. easily recognizable

A. I only
B. I&II
C. I, II, III
D. I, II, III, IV
5. Which division on the geologic time scale is the shortest?
A.eon C. epoch
B.era D. period

6. In which era did humans first appear?

A.Cenozoic C. Mesozoic
B.Paleozoic D. Precambrian

7. Which of the following era is called the “age of reptiles”?

A.Cenozoic C. Mesozoic
B.Paleozoic D. Precambrian
8. If Devonian period is often called the “age of fishes” then, how
about Triassic period?
A.ancient life C. age of mammals
B.age of reptiles D. none of these

9. In his science class, Brent, a Grade 11 STEM student, is

given the task of arranging the correct order of geologic time.
He must determine which of the following sequences is
accurate.
I. era>eon>period>epoch
II. eon>era>period>epoch
III. epoch>period>era>eon
IV. eon>era>epoch>period
A. I B. II C. III D. IV
10. When identifying period under Mesozoic era, Jana
knows that ______________, _______________ and
________________ are correct.
A. Cambrian, Devonian and Jurassic
B. Creataceous, Jurassic and Triasic
C. Paleozoic, Ternary and Quarternary
D. Devonian, Ordovician, Silurian
 FOSSIL MAP
Direction: Copy and complete the concept map below by filling up with the
proper terms listed below. (move up boxes; check capitalization; spacing
 While man divides time into millennia, centuries, decades,
years, months, days, minutes and seconds. Geologists,
specifically geochronologists, tracked down the earth’s history,
measured geologic time and devised the geologic time scale.
What is Geologic Time?
Geologic time is the chronology of the Earth’s formation,
changes, development, and existence. These events are
measured on a geological time scale. Scientists do not measure
geologic time on a clock or calendar. They use a linear timeline
based on the age of rocks and their corresponding fossils as well
as the change in life that occurred over millions of years.

Geologists have divided Earth’s history into a series of time

intervals. These time intervals are not equal in length like the
hours in a day. Instead, the time intervals vary in length. This is
because geologic time is divided using significant events in the
history of the Earth.

How do geologists come up with geologic time? They are able

to come up with the geologic time scale through absolute dating
and relative dating techniques.

How relative and absolute dating were used to determine the

subdivisions of geologic time?

To track down the events that took place in the Earth,

geologists and paleontologists had turned their eyes on the
rocks. In the 1800’s these scientists don’t know yet the ages of
rocks, but one thing is sure they had fully understood relative
ages. Relative dating enables them to determine the relative ages
of rocks based on the stratigraphic principles. In the process of
relative dating, scientists do not determine the exact age of the
rock or the fossils in it. They determine which ones are older or
younger than others. They also saw that fossils in older rocks are
different from the fossils in younger rocks. For example, older
rock layers contain only reptile fossils and younger rock layers
may also contain mammal fossils.

Ordering rock layers from oldest to youngest was a first step in

creating the geologic time scale. It does not only show the order
in which life on Earth changed but also how certain areas
changed over time with regards to climate or type of
 How do index fossils relate to geologic time?
Paleontologists had found out very important
stories in every fossil. Fossils record the
earth’s life stories. One of which is how old the
earth is. Not all fossils will be used to
determine the age of the planet. Only index
fossils or marker fossils or guide fossils are
used to determine the relative age of a rock. A
fossil will only be considered as an index fossil
if it follows the following criteria:

easily recognizable
abundant
widespread
short-lived. 7
One example of an index fossil is the trilobite fossil. It was found
out that trilobites existed during the Paleozoic era from middle
Cambrian period to the Permian period. Other macrofossils that
are considered index fossils are ammonites, brachiopods and
mollusks (see the USGS publication at the end of this module).

Index fossils are able to tell the story of the earth. Aside from
telling us the environmental conditions and the characteristics of
organisms during their existence, the age of a certain rock can
also be determined. The age of rock layers can be determined
through matching. Matching is a process of correlating rock
layers of different location through the presence or absence of a
certain index fossil. Basically, this follows the faunal fossil
succession principle. If you are to study the figure below, the rock
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layers coming from different areas are being matched. The layer
containing the same fossils are basically of the same age and
comes from the same period.
Figure 2 shows how the rock
layers in the first area is being
correlated or matched with
the rock layers in the second
area. As shown, the second
layer from the bottom in the
first area is of the same age as
that of the first layer in the
second area since they
contain the same fossils.
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 The age of an unknown fossil could also be determined through the use of
index fossils. Let us try to look at the picture below where an unknown fossil is
found to be together with known index fossils (mollusks) found at the middle layer
of the rock column. We can say that the age of the unknown fossil starts at the
green layer up to the younger layer because of the presence of its accompanying
fossils.

Figure 3 Using Index Fossils to Match Rock Layers

 Index fossils had helped in determining geological history.
Index fossils are able to tell us the relative age of rocks and even
the age of unknown fossils.

Further, the geological time scale is built up by setting a clear

demarcation of the boundaries between eons, among eras,
among periods and even among epochs. Geologic Time Scale (GTS)
The geologic time scale is the “calendar” for events in Earth’s
history. The enumeration of those geologic time units is
based on stratigraphy, which is the correlation and classification
of rock strata. The index fossil forms that occur in the rocks
provide the chief means of establishing a geologic time scale, with
the timing of the emergence and disappearance of widespread
species from the fossil record being used to delineate the
beginnings and endings of each unit of time in the GTS.
Moreover, absolute dating techniques enables to measure the
length of each unit of time in the GTS.

Unlike the human calendar, the GTS is subdivided into named

units of abstract time arranged in descending order of duration
namely eons, eras, periods, epochs, and ages. Eons being the
longest period and ages being the shortest. The figure shows the
subdivisions in the GTS.
 How are the GTS division decided?

The subdivisions are based on but

not limited to these events:

1.The first appearance of a species.

2.Extinction of key species or mass
extinction events in a short
amount of geologic time.
3.Major global climate change.
4.Formation or separation of
supercontinents.
5.Catastrophic events
6.Shifts in global magnetic polarity
Figure 4. Geologic Time Scale
 We live during the Phanerozoic, which means "visible life." This is the
interval of geological time characterized by abundant, complex fossilized
remains. Being the youngest eon of the geologic time, it is also very well
represented by rock at Earth's surface. Because of these two factors, most
paleontologists and geologists study fossils and rocks from the Phanerozoic
eon.
Era
The smaller time interval that divides an EON. The Phanerozoic
is divided into three eras: Cenozoic, Mesozoic and Paleozoic. Very
significant events in the earth’s history are used to determine the
boundaries of the era.

Most of our knowledge of the fossil record comes from the three
eras of the Phanerozoic eon. The Paleozoic ("old life") era is
characterized by trilobites, the first four-limbed vertebrates and
the ancestors of land plants. The Mesozoic ("middle life") era
represents the "age of dinosaurs," though also is noteworthy for
the first appearances of mammals and flowering plants. Finally,
the Cenozoic ("new life") era is sometimes called the "age of
mammals" and is the era during which we live today.
Periods
Just as eons are subdivided into eras,
eras are subdivided into units of time
called periods. The most well-known of all
geological periods is the Jurassic period of
the Mesozoic era (the movie Jurassic
Park, of course, has something to do with
that).
The Paleozoic era is divided into six
periods. From oldest to youngest, these are
the Cambrian, Ordovician, Silurian,
Devonian,Carboniferous, and Permian.
Note that in the United States, the
Carboniferous is divided into
two separate periods: the Mississippian Figure 4 GTS Subdivisions from Eon to
and the Pennsylvanian. 21 Periods
 Periods of geological time are subdivided into epochs. In turn, epochs
are divided into even narrower units of time called ages.
Some geologists now think that--since humans are having such a
notable impact on the Earth and its life--a new, youngest epoch should
be added to the Quaternary: the Anthropocene. There is still
considerable discussion in the geological community about whether this
epoch should be added, as well as debate about what characteristics
should define its beginning.

The Earth’s Story

Pre-Cambrian Eon
There is nothing much we can gather about this eon since this is the
early years of the Earth’s existence. Moreover, this is a time where there
is or no known living organism. This eon is subdivided into 3 eons that is
why it is also known as the Pre-Cambrian Supereon. Hadean eon is a
time where the planet is so hot that it was named after Hades the Greek
God of the underworld. Archean eon saw the appearance of early forms
of life which became the source of oxygen of the planet. This eon ended
with what is known as the oxygen crisis where increased build up of
oxygen made the planet inhospitable to life. The last of the pre-cambrian
super eon is the Proterozoic eon. This eon is also known as the
Cryptozoic eon or the age of hidden life. Table 1 below summarizes the
event characteristics of this eon.

Table 1 Some Event Characteristics of the Pre-Cambrian Supereon

Time Event characteristics

Hadean 4.6-4.0 BYA •formation of the solar system and its
planets
•formation of early waters in the planet
 Table 1 Some Event Characteristics of the Pre-Cambrian Supereon

Time Event characteristics

Archean 4.0-2.5 BYA •early bacteria and algae start to exist
•early oxygen was produced by prokaryotic
organisms which led to a build up in the
atmosphere
•Oxygen Crisis
Proterozoic 2.5 BYA – 542 •appearance of stable continents
MYA •appearance of bacterial and Archean fossils and
eukaryotic cells
•prokaryotic organisms start to form symbiotic
relationships
•plate tectonics forms Rodinia (the first
supercontinent)
•towards its end autotrophs and soft-bodied
heterotrophs filled up the continental shell of
Rodinia
Phanerozoic Eon
The Phanerozoic eon runs from 542 million years ago to the present.
It is subdivided into three eras namely Paleozoic, Mesozoic and
Cenozoic eras.

The Paleozoic Era started 542 MYA and ended 251 MYA. There
are two important events that took place in this era which are the
Cambrian explosion or the evolution of animal life and the Permian
extinction of which around 90% of the species ceased to exist. It is also
divided into six periods which are the Cambrian period, Ordovician
period, Silurian period, Devonian period, Carboniferous period and
Permian Period.
Cambrian period (542-488 MYA)
Rodinia (the one supercontinent) breaks up into a large land mass
called Gondwana which is now the continents in the southern
hemisphere and fragments of land masses that would now become the
northern continents.
•Earth has no polar ice caps. It is
warm andwet with no distinct seasons.

•Marine life flourished. Many types of

primitive animals called sponges
evolved. Small marine invertebrates
called trilobites are abundant. A
variety of algae also lived in the
oceans.
•There is no known land animals or
plants that existed in this period.
Figure 5 Cambrian Explosion
Ordovician period (488-423MYA)
•Most continents were still part of the supercontinent Gondwana. North
America and northern Europe were slowly moving toward each other.
Near the end of this period, part of Gondwana moved over the South
Pole triggering the ice age.
•The climate started as warm and wet but went colder and an ice age
began.

•The oceans are now filled with different types of invertebrates. New
marine invertebrates with shells evolved and the first corals appeared.
Algae and sponges were dominant in reefs. This period saw the first
fish which were jawless.

•The first land plants that are like mosses and other plants without deep
roots appeared.
•The ice aged that is experienced in this period saw a mass extinction
where around 80 percent of the marine species became non-existent.
Further the abundant glaciers caused sea level to drop.

Silurian period (423-416 MYA)

•Gondwana started moving away from the south pole while North
America and Northern Europe collided.
•The movement of Gondwana resulted in the melting of glaciers which
ended the ice age.
•The climate was cold with glaciers near the South Pole and warmer
near the equator.
•In the oceans, corals appeared, and fish continued to evolve. The first
fish with jaws also appeared during this period.
•Scorpions and millepede-like animals were the first land animals that
came to exist.
•The first vascular plants with special veins to transport liquids
appeared and covered rapidly most of the land surface.

Devonian period (416-359 MYA)

•At the beginning there were three major continental land masses that
moved toward each other. The North America/Europe continent was
near the equator, to the north was a portion of modern Siberia and the
south is Gondwana.
•Sea level was high and much of what is land today was under shallow
seas.

•The climate is warm and mild. The interiors of the large continents are
dry, and salt and gypsum deposits were formed.
• The shallow, tropical seas had abundant reefs and were home to a
variety of marine organisms. Sharks became common. Fishes with lobe
fins evolved. These lobe-finned fishes could breathe when they raise
their heads above the water. The population of trilobites declined due
to an increase in swimming predators.
• Proto-amphibians the very first vertebrates appeared and inhabited
the land.
•The first seed plants evolved. No flowering plants existed at this period.
•At the end of the period, saw the existence of trees and forests.
Carboniferous period (359-299MYA)
•Its name comes from the name Carbon which saw the Earth to be
swampy that is rich in carbon. Gondwana moved close to the North
America/Europe continent causing an initial uplift of the Appalachian
Mountains.
•The early part of the period had a uniform, tropical, wet climate with
little seasonality. There were alternating glacial periods that caused
sea level changes. The land near the equator always stayed moist and
tropical.
•With alternating glacial periods and periodic lowering of sea level
resulted on the disappearance of shallow marine organisms.
•First amphibians evolved to colonize land but had to return to the
water to reproduce.
•The first reptiles appeared and reproduced and laid eggs on land.
•The first land snails and insects with wings appeared.
•Forests were widespread near the equator. The lush plant growth
provided the material for the great coal deposits.
•The first conifers appeared.
Permian period (299-251 MYA)
•Pangaea existed that stretched from the North Pole to the South Pole. Ice
sheets covered the South Pole. There were fewer shallow seas than during the
Carboniferous Period.
•Huge climatic changes occurred due to
the existence of a one giant land mass.
With no moderating effect from bodies of
water, vast deserts were formed in the
central portion of Pangaea. The climate is
dry making the coal swamps dried up.
•As swamps dried up, amphibians
dwindled while reptiles diversified and
spread across the land.
•Plants and animals evolved to adapt to the
dry climate by having waxy leaves and
leathery skins, respectively.
•The period ends with the biggest mass extinction. This is also known as
the “great dying” which is believed to be due to a combination of factors
such as massive volcanic eruption, one or more meteor impacts, global
warming, oceanic conditions and recently animal metabolism due to
increase global temperature.
Mesozoic era is coined from the term “meso” which means middle
animals. This era ranges from 251 MYA to 65.5 MYA which is a time of
dinosaurs. Though, the end of the Paleozoic era was marked by a great
mass extinction, the living things that survived during this era became
the forebears of the major plant and animal groups. It has 3 periods
namely Triassic period, Jurassic period and Cretaceous period.
Triassic period (251 – 200 MYA)
•Tectonic movements caused Pangea to move in different directions at
different rates which resulted to the formation Laurasia (Northern
Hemisphere) and Gondwanaland (Southern Hemisphere).

•The continents were covered with vast deserts and high mountains.
Widespread erosion formed great beds of sandstone in the shallow
seas in and around the continents.

•The climate was hot and dry in the continental interior with no
evidence of ice sheets, becoming more humid and having more rains
in the latter part of the period.

•The dominant land animals were the reptiles. The first dinosaurs
branched off from the reptiles and started to live on land, on air and
in water.
•The first dinosaurs, marine reptiles, lizards and tortoises appeared.
•Mammals appeared but, in a few number, compared with the
dinosaurs.
•Crocodiles were abundant. Insects attained complete
metamorphosis. Modern corals, fish and insects evolved.
•Conifers, cycads and ferns were common. Huge seed ferns and
conifers dominated the forests.
•This ended with a mass extinction of an unknown cause. This
wiped out about thirty five percent of the animal groups.

Jurassic period (200 MYA – 145 MYA)

•The breaking up of Pangaea continued and early Atlantic Ocean and
the Gulf of Mexico appeared as shallow continental seas. Intense
volcanic activity also took place during this period.
•The climate was mild with enough rainfalls.
•This is the “golden age of dinosaurs”. Giant plant-eating dinosaurs
roamed the Earth with smaller vicious carnivores stalking them.
•Flying reptiles and the first birds which were from the flying reptile
ancestors appeared.
•This is also known as the “Age of Cycads” since these plants became
so abundant and diverse. Conifers were the most diverse large trees
at this time.
•Flowering plants appeared for the first time, and new insects also
evolved to pollinate the flowers.
•Mammals evolved but they were of small size.
Cretaceous period (145-65.5 MYA)
•Pangaea continued to break up and the Atlantic Ocean continued to
lengthen and also widen.

•The climate remained mild as in the Jurassic period.

•With the rising sea levels and separated continents, shallow ocean
currents carried warm water farther toward the poles. This created a
mild global climate change.

•Dinosaurs, the ruling group of reptiles, still dominated the land. This
is the time where they reached their maximum size and distribution.
By the end of the period, they died out and mammals start to appear.

•Birds diversified and expanded in numbers.

37
•Many groups of insects appeared and diversified including ants,
termites, bees, butterflies, aphids, and grasshoppers.

•Flowering plants evolved and became diverse which could have been
the reason for the diversification of the insects.

•Another mass extinction occurred at the end of this period which led
to the extinction of the dinosaurs. It is believed that a meteor or a
comet or an asteroid hit the earth which resulted to a massive
tsunami, to darken the skies and stop photosynthesis and
eventually to climate change.

Cenozoic Era means “recent life”. It started 65.5 MYA up to the

present time. There are two periods in this era which are the Tertiary
period and the Quaternary period. The tertiary period is divided into
five epochs while quaternary period into two epochs. Figure 7 below
show the major and climatic changes that occurred during this Era
in each period.
A.Tertiary period

1.Paleocene epoch
Violent earthquakes, volcanic eruptions and mountain buildings
began the formation of the Alps in Europe, the Himalayas in Asia, the
Rockies in North America and the Andes in South America
characterized the period. Flowering plants became dominant on
land. Mammals increased in number and in variety.

2.Eocene epoch
The Alps, Himalayas and Andes continued to grow. The Atlantic
and Indian oceans were formed, presumably through the drifting of
continents. Sea spread in Southern Europe and Northern Africa.
Climate was tropical over much of the earth with glaciers covering
only the tops of high mountains. Complex structures of flowering
plants continued to develop. The earliest horse, camel, rhinoceros,
pig, elephant, cattle and primitive apes (monkeys and gibbons)
appeared.

3.Oligocene epoch
The continents began to grow again. Mountain ranges
continued to build up. A cycle of warm, midland cool seasons
became established. Forests occupied less land, while grassland
increased in area. Grass-eating mammals increased in number
and variety. A primitive, tailless ape, probably the ancestor of
humans appeared.

4.Miocene epoch
Extensive movements of the Earth’s crust joined Asia with
Europe and locked in the Mediterranean Sea. Extensive erosion
started to carve Grand Canyon in North America. Climates were
41

varied, warm in some parts and cooler in other parts. Trees that
seasonally shed leaves increased in number and kind. More
grasslands formed. Fish and mammals steadily increased their
number. Probable primitive humans migrated from Africa to Asia
and Europe. Elephants and horses increased in size. Ducks and
pelicans appeared in shallow rivers and lakes. Primitive penguins
appeared in the Antarctic.

5.Pliocene epoch
Tectonic activities forming Sierra Nevada and the Coast Ranges
in North America began. Subsidence of land formed the North
Sea, the Black Sea, the Caspian Sea and the Aral Sea. The rise of
the Alps, the Himalayas and the Andes continued but more
slowly this time. Climate remained varied as in the Miocene
epoch. Mammals decreased in species. Ape-man, which can be
best describe as pre-human, appeared, and increased in number.
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B.Quaternary period

1.Pleistocene epoch
Glaciers and ice spread and receded several times during the
epoch. The increase of glaciers lowered the ocean level; the melting
of glaciers raised the ocean level. Mammals and primitive people
crossed land bridges exposed by the sinking water level.

2.Recent epoch
Glaciers began to melt, causing the water level to rise again, thus
separating the British Isles from Europe. The climate became
warm; formed more deserts. People developed human-level
intelligence and learned to domesticate animals and cultivate
plants.